1. Field of the Invention
The present invention relates to an optical wavelength identification apparatus, and particularly relates to an optical wavelength identification apparatus, and an optical wavelength identification method, whereby optical wavelength identification error occurring along with temperature change can be accurately corrected.
2. Description of the Related Art
An environment which can reap the benefits of broadband Internet service is being implemented, due to the rapid spread of high-speed access networks employing a band of several to 100 Mbit/s or so, such as recent FTTH (Fiber to the Home), ADSL (Asymmetric Digital Subscriber Line), and so forth, and in order to correspond to increase of communication demand, with a backbone network (core network), extremely large capacity optical communication systems employing the wavelength multiplex technology are now under construction. On the other hand, with a connection portion between a metro network and a core network, the limit of electric switching capabilities still remains, and there is concern that that band bottleneck might occur at this portion. Accordingly, it is conceived as effective means to build new photonic network architecture for directly connecting between an access network and a core network at an optical region without providing an electric switch therebetween, and research and development are being actively pursued in various places. With these optical networks, technique for monitoring of multiple wavelength light components included in light signals and for detecting wavelengths thereof at optical nodes disposed in a core network, metro core network, metro access network, and so forth, is increasingly important.
A configuration example of an optical wavelength detection system will be shown in FIG. 1.
With this configuration example, an optical diffraction device 100, a focusing lens 200, and light receiving means 300 are fixed on a board 400.
The optical diffraction device 100 disperses incident light into light components having multiple wavelengths included in the light thereof. The focusing lens 200 refracts the respective incident light components and focuses these upon the light receiving surface of the light receiving means 300. The light receiving means 300 detect, based on the incident position of the light receiving surface of each light component, the wavelength of the light component thereof. With the configuration example in FIG. 1, light components 40 focused from the focusing lens 200 (FIG. 1 illustrates the optical paths of three light components) are configured so as to temporally change the directions thereof at a reflection mirror 390 to perpendicularly enter the light receiving surface of the light receiving means 300. Here, the example shows a case of employing a PLC (Planner Lightwave Circuit) type spectral element serving as the optical diffraction device 100, and a PD (Photo Diode) array serving as the light receiving means 300.
With the configuration such as FIG. 1, when temperature changes, along with the thermal expansion of the board, the distance between the optical diffraction device 100, the focusing lens 200, and the light receiving means 300, and the focal length of the focusing lens 200 change, and along therewith, the incident positions of the light components focused on the light receiving surface of the light receiving means change, so it is necessary to correct the change thereof.
As for a technology for correcting influence due to such temperature change, with an optical pickup, laser collimator apparatus, and so forth, a technology has been disclosed wherein the focal length change due to temperature is cancelled out by thermal expansion such as the casing of an optical part. Japanese Unexamined Patent Application Publication No. 63-264716 discloses a projection lens for compensating the change of the focal position due to temperature change, Japanese Unexamined Patent Application Publication No. 1-220234 discloses a technology for compensating the focal length change along with the temperature change of a collimator lens, and the relative position change between a light source and the collimator lens due to use of a compensating member, Japanese Unexamined Patent Application Publication No. 10-133087 discloses a technology for compensating the temperature change of the focal position of an optical system, Japanese Unexamined Patent Application Publication No. 2004-281033 discloses a technology for suppressing the amount of astigmatism due to the temperature change, and Japanese Unexamined Patent Application Publication No. 11-64757 discloses a technology for canceling out the change of the focal length due to temperature change.
As described above, with the conventional technologies, correction is made regarding the change caused by the expansion of the board material due to temperature change, and the focal length change of the focusing lens, but the expansion due to the temperature change of a PD array member for detecting light components is not taken into consideration. The conventional correction methods select a material for conforming the lens focal length change due to temperature, and the amount of change due to thermal expansion of the material of the board for fixing between the diffraction device and the focusing lens, and between the focusing lens and the PD array, or a material for approximating the lens focal length change to zero as much as possible, and also approximating the thermal expansion of the member to zero as much as possible. However, the thermal expansion of the PD array itself is not taken into consideration, and accordingly it has been difficult to completely suppress the identification errors of the optical wavelength due to temperature change.
With optical wavelength measurement, an optical wavelength selection switch, and so forth in an optical transmission network, high-precision wavelength identification is demanded, and a higher-precision wavelength identification method for also taking the expansion of the member itself making up the PD array into consideration is demanded.